The present invention relates to a vehicle of the two-wheeled type, in particular a bicycle or more particularly a mountain bike, fitted with rear suspension.
A mountain bike may conventionally comprise two suspensions, a front suspension and a rear suspension. The front suspension generally consists in a telescopic fork. The rear suspension comprises an oscillating assembly and a damping system. The oscillating assembly which supports the rear wheel is connected to the front frame of the bike via one or more pivot connections. The function of the damping system is to suspend the user-supporting frame of the bike elastically while damping shocks due to vertical deflection of the rear wheel. A function of the pivot connection(s) is to guide the vertical movement of said rear wheel that result from irregularities in the terrain.
In a first type of rear suspension, the bottom bracket forms an integral portion of the oscillating assembly so that the suspension has no direct influence on transmission. Regardless of the vertical deflection of the rear wheel, the distance between the bottom bracket and the axis of said wheel remains constant.
In a second type of rear suspension, the bottom bracket is an integral portion of the frame so that the suspension does have an influence on transmission. During vertical deflection of the rear wheel, the distance between the bottom bracket and the axis of the rear wheel varies.
This interaction between the suspension and the transmission can give rise to a so-called xe2x80x9cpumpingxe2x80x9d effect which leads to undesirable displacement of the frame which moves down and up under the effect of energetic pedaling.
Proposals have already been made, in document FR A 2 774 966 for a mountain bike having a rear suspension with two pivot connections and in which the bottom bracket forms an integral portion of the front frame. In that document, the rear suspension is made up of first and second rigid links each connecting the oscillating arm carrying the rear wheel to the front frame, together with a damping system.
Each of the links is preferably made of forged or machined light alloy in the form of two devises connected back to back so as to constitute an H-shape. The cross-bar interconnecting the branches of the H-shape needs to be strongly braced in order to withstand twisting forces adequately without deformation. Two parts of the same type, analogous to the links of document FR 2 774 966 are also to found in document U.S. Pat. No. 5,259,637 under references 60 and 62 and in document FR 2 776 981 under references 7 and 8.
In those examples of known rear suspensions, the links or the like must provide not only the pivot function between the oscillating assembly and the front frame, but they must also provide the mountain bike with strength.
It is also to be observed that the embodiment described in document U.S. Pat. No. 5,259,637 is particularly complex, implementing a very large number of parts connecting the two links indirectly to the damping system which is formed by a compression spring.
The present invention provides a two-wheeled vehicle, in particular a bicycle or indeed a mountain bike, said vehicle being fitted with a rear suspension and comprising a front frame carrying a bottom bracket and an oscillating rear assembly carrying a driving rear wheel, said frame and said rear assembly being connected to each other via two pivot connections and via a damping system. In manner characteristic of the invention, the first pivot connection is formed by a circular eccentric constituted in a casing carrying an eccentric pin, said casing being mounted to pivot in a housing in the front frame, while the rear assembly is fixed to the eccentric pin on either side of the casing; in addition the second pivot connection is formed by a connecting rod constituted by a simple plate disposed in the general direction of the frame.
It is true that proposals have already been made in document EP 0 941 917 to use a circular eccentric disposed in a casing carrying an eccentric pin, however in that document the casing is mounted in a housing in the rear frame and it is the bottom bracket that is mounted on the eccentric pin. In addition, in that case, the second pivot connection is formed by a cam system.
The term xe2x80x9cgeneral direction of the framexe2x80x9d is used to mean the deflection direction of the vehicle when the front wheel is exactly in line with the frame. This is thus a direction which corresponds to the midplane of the frame or which is parallel thereto.
Thus, the rigidity of the rear suspension is provided by the circular eccentric and it is the plate forming the connection rod that allows said suspension to move.
The two functions of providing rigidity and of constituting a linkage are thus dissociated to a large extent in the vehicle of the invention.
The circular eccentric preferably presents eccentricity d1 of the order of 15 millimeters (mm) to 20 mm. In comparison, the length d2 of the connecting rod between its two pivots is of the order of 80 mm to 150 mm.
Advantageously, the circular eccentric is housed in the front frame close to the bottom bracket, and in particular above it and behind it.
The portion of the front frame which is designed to receive the bottom bracket and the circular eccentric is preferably a junction piece made as a single machined part, the unfinished part being obtained by forging, molding, or extrusion. This method of implementation presents several advantages: low manufacturing cost, accurate positioning of the circular eccentric and of the bottom bracket, better overall rigidity. When the front frame is of traditional structure, the junction piece is assembled with the seat tube and with the diagonal tube.
The damping system is fixed firstly to the oscillating rear assembly and secondly to the frame. It combines the functions of damping and of providing suspension. It can be a spring, air, or oil system. For simplification purposes, in the specification below, it is referred to by the generic term of xe2x80x9cdamperxe2x80x9d. The cylinder of the damper is fixed to the frame and the end of its rod is fixed to the oscillating rear assembly. The damping system acts in a direction which corresponds to the longitudinal axis of the cylinder and the rod of said damper. The damping system preferably acts in a direction that makes a small angle xcex1 relative to the portion of the frame to which said system is fixed, where xcex1 is advantageously less than 30xc2x0, and is preferably about 20xc2x0. This particular disposition has the effect of considerably reducing the stresses on and the deformation of the front frame. In particular, it enables the damping system to be fixed to the cross-bar of the frame, assuming that it is a traditional frame having a substantially horizontal cross-bar, a seat tube, and a diagonal tube. Under such circumstances, the angle xcex1 corresponds to the angle between the general direction of the cross-bar and the axis of the damping system; when this angle xcex1 is less than 30xc2x0, there is no need to give additional rigidity to the cross-bar of the frame. Clearly the angle xcex1 is caused to vary as the suspension moves through an angle, thereby compressing the damping system, however such variation remains within the limits stated above, i.e. the angle is less than or equal to about 30xc2x0.
In a preferred variant embodiment, the damping system, the oscillating rear assembly, and the plate forming a connecting rod are secured to one another about a common pivot pin. This thus provides a common fixing for the connecting rod and for the damping system with the oscillating rear assembly, thereby reducing the number of pivots that need to be made and also the number of assemblies, and thus also achieving a significant saving of weight.
In a preferred embodiment, the damping system acts substantially in line with a rectilinear portion of the oscillating rear assembly having a first end fixed to said damping system and a second end carrying the rear wheel. This particular disposition provides optimum transmission of forces from the rear wheel on the damping system.
The oscillating rear assembly is preferably of a substantially triangular configuration, being formed by a chain-stay rod between the eccentric pin of the eccentric casing and the rear wheel, a seat stay between the rear wheel and the connecting rod pivot, and a reinforcing rod between the connecting rod pivot and the eccentric pin of the eccentric. Increased rigidity is obtained because the triangle is closed. In this configuration, as mentioned above, the damping system preferably acts substantially in line with the seat stay.